What is "ballerburg"

Ballerburg is a turnbased Artillery game from 1987, written in C by Eckhard Kruse for the Atari ST. The game was distributed free of charge as public domain software, but described itself as " Donationware", the author asking for a donation of 20 DM, offering as incentive the source code for the game. The game's monochromatic graphics and sound were up to standard for that time.